1. Field of the Invention
The present invention relates to an optical receiving device adapted to receive an optical signal and, more particularly, to an optical receiving device adapted to receive a wavelength multiplexed optical signal.
2. Description of the Related Art
Conventionally, 1 Gbps or 2.4 Gbps optical receiving devices have been most common. Technological advances have now resulted in successive announcements of novel technologies for 100 Gbps devices and the transmission rate of optical communication devices has been increased accordingly. There is also a high demand for miniaturization of devices as well as improvements in transmission rate. Small-sized optical transceivers such as XFP or SFP have rapidly become popular in place of 300pinMSA that has been in the main stream conventionally. In order to fully exploit the capabilities of small-sized and high-speed optical transceivers, the devices need be adjusted optimally. One of the parameters required to be adjusted is receiver decision threshold voltage (RXDTV).
RXDTV defines a threshold level for distinction between “0” and “1” of received signals. RXDTV need be maintained at an optimal level in order to take the maximum advantage of the specification of an optical receiver defined in Multi-Source Agreements standards.
FIG. 1 is a graph showing the relationship between RXDTV and the error rate. As shown in FIG. 1, as RXDTV is changed, the error rate is changed accordingly. The optimal value of RXDTV varies depending on individual optical receivers. The value of RXDTV occurring when the error rate is at minimum is the optimal value. It should also be noted that RXDTV is affected by the condition of a transmission line. FIG. 2 is a graph showing an exemplary relationship between the cumulative amount of dispersion in optical fiber transmission and the optimal value of RXDTV. The eye opening of an optical signal is distorted due to the impact from dispersion so that the optimal value of RXDTV varies. RXDTV also varies depending on the temperature, optical input level, and wavelength. It is therefore desired to control RXDTV at the optimal value in an optical receiver.
Methods of controlling RXDTV to the optimal value are known in the related art. For example, in one known method (cited document 1), RXDTV of an optical receiver is controlled to the optimal value after superimposing a non-modulated light on an optical signal transmitted over a long distance.    [patent document 1] JP2006-60640
However, no methods have been proposed that are adapted for an optical receiving device for receiving a wavelength multiplexed optical signal in which optical signals of a plurality of wavelengths are multiplexed and that are capable of suitably adjusting RXDTV of optical receivers for receiving respective optical signals.